1. Field of the Invention
The present invention relates to circuitry for terminating electrical signals. More specifically, the present invention relates to circuitry for terminating a ring signal of a telephone network.
2. Present State of the Art
From its inception, the design of telephones and telephone networks focused on transmitting the human voice, which generates frequencies that are generally less than 20 kHz. Accordingly, the telephone network was designed to function with these frequencies in mind. These frequencies proved to be limiting factors, however, with regard to newer technologies. Modems, for instance, can be designed to transfer data at extremely high rates, but they are effectively limited to the much lower frequencies that the telephone system can handle. Data transfer speeds can be increased using compression techniques, but in a general sense, the speed at which data can be communicated across a telephone line by a modem is limited by the technology of the telephone network.
As the telephone system continued to develop, standards were created which dictated the requirements to be met by devices connecting to the telephone network. For instance, the telephone company places a direct current voltage of approximately xe2x88x9248 volts on the telephone line, but requires that the connecting device draw little or no current when the device is not being used. This requirement is met by designing the device to present a large load to the telephone system. When these and other specifications are satisfied, the connecting device functions properly and problems such as signal reflection can be avoided.
In addition to transferring voice data, telephone networks use a ring signal to notify a telephone or other device that they have an incoming communication. The circuitry which recognizes and terminates the ring signal is designed to meet certain specifications. Because the receiving device is still on hook when the ring signal is received, the circuitry first detects the ring signal. The ring signal also has a lower frequency than voice signals and the receiving device should be able to properly terminate the ring signal. Third, once the receiving device recognizes the ring signal and goes off hook, the circuitry which terminates the ring signal should not interfere with the voice or data transmissions.
These requirements can be met for a particular telephone network, but designing a telephone or other device that meets the ring termination impedance requirements for a number of different telephone networks is a difficult task for several reasons. There are many different telephone networks in the world and many of them specify a different ring termination impedance. This immediately presents a problem because a telephone or other device with a single physical ring termination impedance that is designed for a particular telephone network, creates signal reflections and other problems when the device connects with other telephone networks.
The inability to accurately match the ring termination impedance of more than one network can be solved by inserting multiple ring termination impedances and using electronic switches or relays to connect the device to the appropriate ring termination impedance. The obvious disadvantage of this method is the cost of the necessary parts and the cost of the manufacturing process. Additionally, modems and other devices embodied on PCMCIA compliant cards have limited surface area. Physically placing the necessary switches, relays and impedances on the printed circuit board of a PCMCIA card may substantially reduce the surface area available for other card components and may prohibitively increase the cost of the card. It is desirable to have circuitry that can match the ring termination impedances of different telephone networks in a less expensive and more efficient manner.
Thus, it would be an advance in the art to provide a low cost and easily configurable method and circuit for appropriately terminating a ring signal in accordance with specifications of a specific telephone network.
It is an object of one embodiment of the present invention to substantially match the ring termination impedance specified by a telephone network.
It is another object of one embodiment of the present invention to synthesize an impedance which effectively terminates a ring signal.
It is yet another object of one embodiment of the present invention to effectively match the ring termination impedances of one or more telephone networks.
It is a further object of one embodiment of the present invention to reduce the circuit components necessary to effectively match the ring termination impedances of one or more telephone networks.
It is another object of one embodiment of the present invention to combine a synthesized impedance with a device impedance in order to match a ring termination impedance.
It is yet another object of one embodiment of the present invention to not draw direct current from the telephone network.
In summary, the basic problem solved by the present invention is being able to effectively match the ring termination impedances of the ring signals specified by telephone networks. One embodiment of the present invention eliminates the need to have a plurality of physical impedances which are connected to the telephone network using relays and switches. Instead, a terminating impedance is synthesized. The synthesized impedance is connected in a parallel configuration to the physical device impedance. The parallel configuration of the device impedance and the synthesized impedance substantially matches the specified ring termination impedance.
The synthesized impedance is created by measuring the line voltage of the telephone line and generating a current. The line voltage divided by the generated current equals the synthesized impedance. In order to synthesize an impedance that will produce the ring termination impedance when combined with the device impedance, the magnitude of the current is controlled. In this manner the synthesized impedance can alter the device impedance such that the ring termination impedance of one or more telephone networks may be effectively matched.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.